1. Field of the Invention
The present invention relates to an image processor for producing a special effect, particularly cross fade, during the visual display of image data.
2. Description of the Prior Art
One special effect known as "dissolve (cross fade)" is commonly used in the display of still images, for example, on a monitor screen. This special effect exhibits, rather than an immediate shift from one picture to another, a so-called cross fading in which the current image is faded out while a subsequent image is faded in. If the data of the current image is denoted A, the data of the subsequent image is denoted B, and a control signal is denoted .alpha., the "dissolve" effect is expressed as: EQU C+A.multidot..alpha.+B.multidot.(1-.alpha.)
where the effect is realized by gradually varying .alpha. from 1 to 0. This visual effect becomes feasible using hardware such as a multiplier or a ROM having a lookup table. With respect to circuitry simplicity and high-speed operation, the use of a ROM is more advantageous. A technique using the ROM will now be described.
The image data of the current and subsequent images and the control signal .alpha. are fed to the ROM and together serve as an address signal of the ROM. Then, the ROM produces two resultant outputs denoting the products A.multidot..alpha. and B.multidot.(1-.alpha.) corresponding to previously stored data. A.multidot..alpha. and B.multidot.(1-.alpha.) are then summed to obtain intermediate image data C denoting a cross faded image.
If each original image data A and B consists of an 8-bit word and the control signal .alpha. consists of a 5-bit word, the ROM must accept a 13-bit address signal and thus, must at least have a memory capacity of 8 Kbits. Since the control signal .alpha. contains 5-bits, the intermediate image data C is produced in the form of 32 gradations; 31/31, 30/31, . . . , 1/31, and 0/31, corresponding to the variation of .alpha. from 1 to 0. If each of the resultant intermediate image data C is displayed for 1/30 second, the total duration of the display shift from the current image A to the subsequent image B as .alpha. varies from 1 to 0 takes a period of 1/30.times.31 second or about one second. The longer the display shift duration, the more noticeable the shift from the image data A to the image data B will be. However, if the duration of the display shift is increased by extending the display time of each intermediate image, the "dissolve" effect will appear jerky (not smooth). For increasing the duration of the "dissolve" effect and at the same time exhibiting a smooth visual effect, it is necessary to increase the number of bits of the control signal .alpha.. However, this would require that the memory capacity of the ROM in a prior art image processor also be increased. Also, if the case where a multiplier is employed, the number of bits of a multiplication coefficient would have to be increased and thus, an arithmetic operation at a higher speed would be necessary.